Discriminators for f.m. noise measurements require high Q resonators. This need was partially circumvented by the transmission line discriminators of U.S. Pat. No. 4,002,970 and 4,002,971. U.S. Pat. No. 3,675,124 which taught the use of cavity resonators indicates the desired properties for a resonator used in a disciminator. It must have high Q; and it must be matched to the transmission line at resonance. The disciminator element of U.S. Pat. No. 4,002,970 cannot be operated as a cavity because of the electrical shortcomings of the slide screw tuner. As the transmission line is made shorter and the loss reduced, more insertion of the tuner screw is required. Since this tuner can produce only a VSWR of about 20, it will not serve to match a short transmission line to give an optimum cavity Q.